A propellant is understood to be a substance, usually a mixture of fuel and oxidizer for propelling an object including, without limitation, a rocket. High triple point propellants are useful fuels in the field of rocketry. The triple point of a substance is understood to be the temperature and pressure at which the three phases of that substance (solid, liquid and gas) coexist in thermodynamic equilibrium. High triple point propellants have their triple point above ambient pressure or above the pressure of an engine combustion chamber at the time of engine ignition.
In the field of rocketry, the propellants used during engine ignition are subjected to significant thermodynamic challenges as they proceed from on-board propellant storage pressures and temperatures, typically in a fuel tank, and into the combustion chamber of a rocket engine. Certain so-called high triple point propellants have gained interest due to their suitable performance in rocket engine systems. However, high triple point propellants present challenges relative to their use in rocket-based missions, including missions in space.
For example, at engine start up, the combustion chamber of a rocket engine is at a lower pressure (usually near ambient pressure) than the storage pressure of the propellant. If a propellant has a triple point higher than the ambient pressure, the state of the liquid propellant will begin to change from liquid to a gas and solid. If a solid phase forms during ignition, such propellant in solid form can interfere with a desired steady flow of propellant into the combustion chamber of a rocket engine. For example, plugging of injection openings into an engine may occur, interfering with optimum flow, or causing oscillated (e.g. uneven) flow. Both plugging of the injector and oscillated flow of propellant into an engine, especially during start up (e.g. ignition), can cause significant problems relative to an ignition sequence regimen.
Mechanical methods have been attempted to physically block or partially block propellant flow through an engine nozzle throat, in an attempt to achieve a pressure within an engine chamber that maintains a high triple point propellant in its desired phase as propellant flow is established into the combustion chamber of a rocket engine. Such mechanical methods include mechanical plugs inserted into the throat of an engine (e.g. burst disc, throat plug). Such mechanical devices are often set at a predetermined pressure setting that is influenced by ambient temperature. In other words, if the temperature environment of such device varies, the set pressure also varies. In addition, once such mechanical devices “release” (e.g. rupture, etc.), the flow rate of propellant into the combustion chamber dramatically increases, potentially causing the combustion chamber pressure to vary; possibly dropping below the propellant's triple point. In addition, such devices used to build pressure and then release are single use devices, and thus do not address the problem of relight of the engine, such as, relight of a rocket engine at a later point in the mission in space, etc.